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By Grounding

First charge the glass vessel with oil of turpentine, with a camel-hair pencil, and while moist apply the color in a dry powder, which will adhere to the oil; or, instead of oil of turpentine, use any of the materials used for printing; but the first is the best.

Cautions to be Observed in making Colored Enamels.

In making these enamels, the following general cautions are necessary to be observed. 1st. That the pots be glazed with white glass, and be such as will bear the fire. 2d. That the matter of enamels be very nicely mixed with the colors. 3d. When the enamel is good, and the color well incorporated, it must be taken from the fire with a pair of tongs.

General Method of making Colored Enamels.

Powder, sift, and grind all the colors very nicely, and first mix them with one another, and then with the common matter of enamels; then set them in pots in a furnace, and when they are well mixed and incorporated, cast them into water, and when dry set them in a furnace again to melt, and when melted take a proof of it. If too deep colored, add more of the common matter of enamels; and if too pale add more of the colors. To Obtain Black Enamel with Platina.

Mix some chloride of platina, dissolved in water, with neutral nitrate of mercury, and expose the precipitate, which will be formed, to a heat simply sufficient to volatilize the proto-chloride of mercury; there will be obtained a black powder, which, applied with a flux, gives a beautiful black enamel.

To make Enamel, called Niello.

Take 1 part of pure silver, 2 of copper, and 3 of pure lead, fuse them together, and pour the amalgam into a long-necked earthenware matrass, half filled with levigated sulphur; let the mouth of the vessel be immediately closed, and the contents left to cool. The mass which results, when levigated and washed, is ready for the purposes of the artist. The cavities left by the fusion having been filled with it, the plate is to be held over a small furnace, fed with a mixture of charcoal and wood, taking care to distribute the enamel with the proper instrument. As soon as fusion has taken place, the plate is to be removed; and, when sufficiently cooled, is to be cleared by the file, and polished by fine pumice and tripoli.

To Paint in Enamel.

The enamel painter has to work, not with actual colors, but with mixtures, which he only knows from experience will produce certain colors after the delicate operation of the fire; and to the common skill of the painter, in the arrangement of his palette and choice of his colors, the enameller has to add much practical knowledge of the chemical operation of one metallic oxide on another; the fusibility of his materials; and the utmost degree of heat at which they will retain, not only the accuracy of the figures which he has given, but the precise shade of color which he intends to lay on.

Painting in enamel requires a succession of firings; first of the ground which is to receive the design, and which itself requires two firings, and then of the different parts of the design itself.

The ground is laid on in the same general way as the common watch-face enamelling. The colors are the different metallic oxides, melted with some vitrescent mixture, and ground to extreme fineness. These are worked up with an essential oil (that of spikenard is preferred, and next to it oil of lavender) to the proper consistence of oil colors, and are laid on with a very fine hair brush. The The essential oil should be very pure, and the use of this rather than of any fixed oil, is that the whole may evaporate completely in a moderate heat, and leave no carbonaceous matter in contact with the color when red hot, which might affect its degree of oxidation, and thence the shade of color which it is intended to produce. As the color of some vitrified metallic oxides (such as that of gold) will stand at a very moderate heat, whilst others will bear, and even require a higher temperature to be properly fixed, it forms a great part of the technical skill of the artist to supply the different colors in proper order; fixing first those shades which are produced by the colors that will endure the highest, and finishing with those that demand the least, heat. The outline of the design is first traced on the enamel, ground and burnt in; after which the parts are filled up gradually by repeated burnings, to the last and finest touches of the tenderest enamel.

Transparent enamels are scarcely ever laid upon any other metal than gold, on account of the discoloration produced by other metals. If, however, copper is the metal used, it is first covered with a thin enamel coating, over which gold-leaf is laid and burnt in, so that, in fact, it is still this metal that is the basis of the ornamental enamel.

To Manufacture Mosaic as at Rome.

Mosaic work consists of variously shaped pieces of colored glass enamel; and when these pieces are cemented together, they form those regular and other beautiful figures which constitute tessellated pavements. The enamel, consisting of glass mixed with metallic coloring matter, is heated for 8 days in a glass-house, each color in a separate pot. The melted enamel is taken out with an iron spoon and poured on polished marble placed horizontally; and another flat marble slab is laid upon the surface, so that the enamel cools into the form of a round cake, of the thickness of three-tenths of an inch.

In order to divide the cake into smaller pieces, it is placed on a sharp steel anvil, called tagliulo, which has the edge uppermost; and a stroke of an edged hammer is given on the upper surface of the cake, which is thus divided into long parallelopipeds, or prisms, whose bases are threetenths of an inch square. These parallelopipeds are again divided across their length by the tagliulo and hammer into pieces of the length of eight-tenths of an inch, to be used in the Mosaic pictures. Sometimes the cakes are made thicker and the pieces larger.

For smaller pictures, the enamel, whilst fused, is drawn into long parallelopipeds, or quadrangular sticks; and these are divided across by the tagliulo and hammer, or by a file; sometimes, also, these pieces are divided by a saw without teeth, consisting of a copper blade and emery; and the pieces are sometimes polished on a horizontal wheel of lead with emery.

Gilded Mosaic.

Gilded Mosaic is formed by applying the goldleaf on the hot surface of a brown enamel, immediately after the enamel is taken from the furnace; the whole is put into the furnace again for a short time, and when it is taken out the gold is firmly fixed on the surface. In the gilded enamel, used in Mosaic at Rome, there is a thin coat of transparent glass over the gold.

On the Different Glazes used for Cooking Utensils.

The wrought and cast-iron vessels which are to be placed on the fire are often covered with enamel, which protects the liquid from metallic contact with the sides. Two compositions are generally employed for this purpose, one having for its base silicate of lead, and the other boro-silicate of soda. These enamels are applied to the scoured surface of the metal in the form of a powder, which is fixed by heating it to a sufficiently high temperature to fuse it; it then spreads over and covers the metal with | nitric acid, which immediately communicates a a vitreous varnish.

The boro-silicate of soda enamel possesses great superiority over that of silicate of lead, for it is unattacked by vinegar, marine salt, the greater number of acid or saline solutions, even when concentrated, and resists the action of the agents employed in cooking or chemical operations.

The silicate of lead enamel is whiter and more homogeneous, which explains the preference given to it by the public, but it gives up oxide of lead to vinegar or to common salt; it acts upon a great number of coloring matters, and it is attacked by dull appearance to it. On evaporation the liquid leaves a white crystalline residue of nitrate of lead. This enamel is instantly darkened by dissolved sulphides, and also by cooking food containing sulphur, such as cabbage, fish, and stale eggs. It is very easy to distinguish these two enamels by means of a solution of sulphide of potassium, sodium, or ammonium. On allowing of one of these reagents to fall on the vessel to be tested, the lead enamel darkens in a few moments, whilst the boro-silicate of soda enamel retains its white color.